Pulsating combustion is a phenomenon which is well known in the art of air and water heating and is quite extensively discussed in the literature. Early examples of prior art pulse combustion apparatus are shown in U.S. Pat. Nos. 2,898,978 (issued to the present inventor--John A. Kitchen) and 3,005,485 (Salgo and Kitchen). Other examples of United States patents that have issued to John A. Kitchen in this field are U.S. Pat. Nos. 2,916,032; 3,267,985; 4,241,720; 4,241,723; and 4,309,977.
Conventional dehydration processes generally involve moving the material through a zone of heated air which is being blown at high velocity. After drying, the material is separated from the air and collected. In conventional flash driers, material being dried is conveyed pneumatically and dried at the same time through a long duct or pipe. This system is particularly advantageous in drying particulate and combustible substances. Air and gas velocities may have to be as high as 5000 ft/min or more in order to transport the material; as such, considerable electrical energy must be expended to drive blowers. This is in addition to the fuel necessary for the heat required for drying.
It has been recognized that the rate of drying can be increased by imposing sound waves on the material in the presence of heated air. Pulsating combustion has been suggested as a method of providing intense sonic waves for dehydration, as well as the required heat and high velocity gas stream.
U.S. Pat. Nos. 4,706,390 and 4,832,598 (both to John A. Kitchen) disclose pulse combustion dehydrators. Other examples are shown in Lockwood's U.S. Pat. Nos. 3,462,955 and 3,618,655. Also of interest are U.S. Pat. No. 4,699,588 (Zinn et al.) and German Offenlegungschrift 28 10 045 (Piterskich et al.).
A pulse combustor consists essentially of a combustion chamber which has one-way air and fuel inlet valves at one end and, at the other end, a tailpipe through which exhaust gases are expelled. This expulsion results from the force of cyclic explosions of an air/fuel mixture. Following each explosion there is a partial vacuum in the combustion chamber which draws in another charge of fuel and air. Exhaust gases are also drawn back into the chamber. Flame fronts in the returning exhaust gases cause ignition of the new charge. Interference with this wave delays or prevents ignition of the new charge and can weaken or completely stop the combustion cycle.
It has been proposed to inject the material to be dried directly into the tailpipe of the pulse combustor in order to maximize the heat and pressure of the exhaust gases on the material. Interference with the combustion cycle is prevented by injecting the material into an enlarged section of pipe where the velocity of the gases provide a low pressure zone (see e.g. the '598 Kitchen patent referred to previously). Material will actually be drawn into the tailpipe at this point without assistance, although a feeding mechanism is usually provided to control the rate of flow.
In existing applications of pulse combustion to drying, the material is discharged from the tailpipe of the pulse combustor into a rotary drum or other large volume where circulating exhaust gases dry the material.